The reading of electrical energy, water flow, and gas usage has historically been accomplished with human meter readers who came on-site and manually documented meter readings. Over time, this manual meter reading methodology has been enhanced with walk by or drive by reading systems that use radio communications to and from a mobile collector device in a vehicle. Recently, there has been a concerted effort to accomplish meter reading using fixed communication networks that allow data to flow from the meter to a host computer system without human intervention.
Fixed communication networks can operate using wire line or radio technology. For example, distribution line carrier systems are wire-based and use the utility lines themselves for communications. Radio technology has tended to be preferred due to higher data rates and independence from the distribution network.
Automated systems, such as Automatic Meter Reading (AMR) and Advanced Metering Infrastructure (AMI) systems, exist for collecting data from meters that measure usage of resources, such as gas, water and electricity. Such systems may employ a number of different infrastructures for collecting this meter data from the meters. For example, some automated systems obtain data from the meters using a fixed wireless network that includes, for example, a central node, e.g., a collection device, in communication with a number of endpoint nodes (e.g., meter reading devices (MRDs) connected to meters). At the endpoint nodes, the wireless communications circuitry may be incorporated into the meters themselves, such that each endpoint node in the wireless network comprises a meter connected to an MRD that has wireless communication circuitry that enables the MRD to transmit the meter data of the meter to which it is connected. The wireless communication circuitry may include a transponder that is uniquely identified by a transponder serial number. The endpoint nodes may either transmit their meter data directly to the central node, or indirectly though one or more intermediate bi-directional nodes that serve as repeaters for the meter data of the transmitting node.
Some networks may employ a mesh networking architecture. In such networks, known as “mesh networks,” endpoint nodes are connected to one another through wireless communication links such that each endpoint node has a wireless communication path to the central node. One characteristic of mesh networks is that the component nodes can all connect to one another via one or more “hops.” Due to this characteristic, mesh networks can continue to operate even if a node or a connection breaks down. Accordingly, mesh networks are self-configuring and self-healing, significantly reducing installation and maintenance efforts.
Within these smart mesh networks, communications are commonly conducted in the 900 MHz Industrial, Scientific, and Medical (ISM) frequency band, which uses frequencies in the 902-928 MHz frequency range. Radio frequency (RF) technology in this range can operate without a Federal Communications Commission (FCC) license by restricting power output and by spreading the transmitted energy over a large portion of the available bandwidth. Many systems are designed by communications experts rather than by metering or utility experts. As a result, many such systems are very complex, require heavy synchronization traffic, and are difficult to troubleshoot.
While many mesh networks can perform on demand reads from the collector, the default mode of operation is ad hoc, in that endpoint devices establish connections with neighboring devices and initiate data transmissions directed to the collector. To eliminate contention within an ad hoc mesh communication system, sophisticated synchronizing schemes are required to ensure both frequency and time are kept extremely accurately. Endpoint devices need to maintain their synchronization to know exactly when to transmit and on what frequency to transmit. If tight frequency and time synchronization are not maintained, system throughput is drastically and adversely impacted.
Polled type mesh networks, such as the EnergyAxis network available from Elster Electricity, LLC, use self-coherent communications at all endpoint devices within the network and do not rely on frequency or time synchronization by the endpoint devices. This is because the endpoint devices do not need to know the time or frequency of an expected message. Each endpoint device in a polled type mesh network looks for incoming messages continually and can synchronize to both time and frequency on the fly by decoding the preamble of an incoming message. By avoiding the need for frequency and time synchronization, the complexity of the endpoint device and of the network as a whole can be reduced. The amount of communications traffic on the network can be reduced by nearly an order of magnitude.
In a polled type mesh network, an endpoint device normally only outputs data when it is asked for information or when it has an exception message based on a change of state, alarm condition, or scheduled event. In the EnergyAxis system, several seconds are periodically set aside for exception traffic. All exception messages are decoded on the fly by endpoint meters that need to forward the data on to the collector.
Polled type mesh networks generally use polled messages and exception messages that have a fixed length. Such messages typically include a preamble time that is long enough for the endpoint device to scan all of the possible channels in order to find the channel on which the message is transmitted. After this channel is found, an algorithm is executed to accurately lock on the preamble bits and decode them until a start frame delimiter is received. The start frame delimiter marks the end of the preamble and the start of the data portion, or payload, of the message. In some cases, a different start frame delimiter is used to indicate one data rate as opposed to another. The data portion of the message, sent on the same frequency, is decoded to determine the desired function. Each new message from a given device is transmitted on a new frequency to promote the equal utilization of spectrum that is required by FCC regulations for 900 MHz ISM band operation. In addition, the new frequency complies with some pseudorandom sequence of channels.
Under current FCC regulations, a system with 25 frequency channels can transmit with up to 0.25 W power. A system with 50 frequency channels, on the other hand, can transmit with up to 1 W power. Accordingly, increasing the number of frequency channels would allow a mesh network to transmit with more power. However, as the number of channels increases, the length of the preamble required to ensure that a particular channel is found also increases.